Temperature performance of AlGaN/GaN MOS-HEMTs on Si substrates using Gd2O3 as gate dielectric

GaN based high electron mobility transistors have draw great attention due to its potential in high temperature, high power and high frequency applications [1, 2]. However, significant gate leakage current is still one of the issues which need to be solved to improve the performance and reliability of the devices [3]. Several research groups have contributed to solve this problem by using metal–oxide–semiconductor HEMTs (MOSHEMTs), with a thin dielectric layer, such as SiO2 [4], Al2O3 [5], HfO2 [6] and Gd2O3 [7] between the gate and the barrier layer on AlGaN/GaN heterostructures. Gd2O3 has shown low interfacial density of states(Dit) with GaN and a high dielectric constant and low electrical leakage currents [8], thus is considered as a promising candidate for the gate dielectrics on GaN. MOS-HEMTs using Gd2O3 grown by electron-beam heating [7] or molecular beam epitaxy (MBE) [8] on GaN or AlGan/GaN structure have been investigated, but further research is still needed in Gd2O3 based AlGaN/GaN MOSHEMTs.

Stacked GaAsSbN-capped InAs/GaAs quantum dots for enhanced solar cell efficiency

Different approaches have recently arisen aiming to exceed the Shockley-Queisser efficiency limit. Particularly, the use of self-organized quantum dots (QD) has been recently proposed in order to introduce new states within the barrier material, which enhances the subband gap absorption yielding a photocurrent increase. Stacking QD layers allows exploiting their unique properties for intermediate-band solar cells (SC) or tandem cells.In all these cases, tuning the QD properties by modifying the capping layer (CL) can be very useful.

Laser induced forward transfer of silver pastes for printing of fingers in c-si cells.

The main objective of this work is to adapt the Laser Induced Forward Techniques (LIFT), a well- known laser direct writing technique for material transfer, to define metallic contacts (fingers and busbars) onto c-Si cells. The silver paste (with viscosity around 30-50 kcPs) is applied over a glass substrate using a coater. The thickness of the paste can be control changing the deposit parameters. The glass with the silver paste is set at a controlled gap over the c-Si cell. A solid state pulsed laser (532 nm) is focused at the glass/silver interface producing a droplet of silver that it is transferred to the c-Si cell. A scanner is used to print lines. The process parameters (silver paste thickness, gap and laser parameters -spot size, pulse energy and overlapping of pulses) are modified and the morphology of the lines is studied using confocal microscopy. Long lines are printed and the uniformity (in thickness and height) is studied. Some examples of metallization of larger areas (up to 10 cm x 10 cm) are presented.

Estudio de la estabilidad genética asociada a los procesos de crioconservación de ápices de menta

La crioconservación se ha descrito como una técnica de conservación ex situ a largo plazo que ha sido aplicada con éxito a numerosas especies, y resulta especialmente importante en aquellas con propagación vegetativa, infértiles o amenazadas, en las que sistemas de conservación ex situ más sencillos, como los bancos de semillas, no son posibles. También presenta ventajas frente a la conservación in vitro, ya que logra disminuir o eliminar problemas como la excesiva manipulación del material, evitando los subcultivos periódicos y disminuyendo así el riesgo de contaminaciones y de aparición de variación somaclonal. Sin embargo, someter al material vegetal a los procedimientos que implica la crioconservación provoca distintos estreses. Entre ellos, el estrés oxidativo puede potencialmente producir daños en membranas, proteínas, carbohidratos y en el ADN. En este trabajo se han evaluado diversos sistemas de crioconservación en ápices de Mentha × piperita L., híbrido estéril entre Mentha aquatica L. y Mentha spicata L. Se han utilizado ápices de dos genotipos (‘MEN 186’y ‘MEN 198’) en los cuales se compararon dos técnicas de crioconservación, encapsulación-deshidratación y vitrificación-droplet. El análisis de la supervivencia y capacidad de regeneración del material sometido a los tratamientos de crioconservación, junto con el análisis de la estabilidad genética de dicho material mediante marcadores moleculares (RAPD y AFLP) han permitido comparar los distintos protocolos y tratamientos establecidos. El estudio sobre el tipo de protocolo empleado reveló una mayor variabilidad genética en la técnica de encapsulación-deshidratación, especialmente en el genotipo ‘MEN 186’, ya que ‘MEN 198’ resultó ser más estable en todos los análisis. La inestabilidad encontrada en esta técnica no fue exclusiva de aquellos explantos crioconservados, sino que los pasos previos a la inmersión en nitrógeno líquido (NL) también provocaron variaciones en el ADN. Según el tipo de muestra analizada se encontraron diferencias en la estabilidad: muestras provenientes de callos presentaron una mayor inestabilidad que aquellas de hojas (brotes). Se utilizaron tres medios para la recuperación de los ápices tras la crioconservación con el uso de diferentes combinaciones de reguladores de crecimiento: “Reed” (0,5 mgL-1 6-bencilaminopurina, BAP), “Senula” (0,5 mgL-1 6-dimetilalilamino-purina, 2-iP + 0,1 mgL-1 ácido α-naftalen-acético, ANA) y “Nudos” (0,5 mgL-1 BAP + 0,1 mgL-1ANA). El medio “Reed” produjo un aumento en la supervivencia y recuperación de los ápices en ambos genotipos y técnicas, y disminuyó la formación de callo. Sin embargo, no tuvo un efecto significativo en la estabilidad genética. El medio “Senula” provocó una mayor estabilidad genética en el genotipo más inestable, ‘MEN 186’. Para reducir el daño oxidativo producido durante la encapsulación-deshidratación, e incrementar la recuperación de los ápices manteniendo su estabilidad genética, se comparó el efecto de añadir sustancias antioxidantes en el precultivo de los ápices (ácido ascórbico, vitamina E y glutatión). No se obtuvo la respuesta esperada y estos tratamientos no presentaron efectos significativos tanto en la estabilidad como en la recuperación. Para entender mejor qué sucede durante todo el proceso de encapsulación-deshidratación, se evaluó cada paso del protocolo por separado y su efecto en la estabilidad y la recuperación. Además, se determinó el estado de oxidación en cada etapa mediante la cuantificación de malondialdehído y la detección de la formación de radicales libres (mediante el ensayo del ácido tiobarbitúrico, y sondas fluorescentes específicas, respectivamente). Se determinó que a partir de los primeros pasos se genera estrés oxidativo, el cual aumenta a medida que se avanza por el protocolo hasta la inmersión en nitrógeno líquido. Esto se ve reflejado en la disminución progresiva tanto de la recuperación como de la estabilidad genética. Con el uso de antioxidantes en el precultivo (ácido ascórbico y vitamina E) no se obtuvo un efecto positivo en el mantenimiento de la estabilidad genética, y tan sólo con el uso de vitamina E se observó una recuperación mayor en uno de los pasos estudiados (después de la desecación). Sin embargo, cuando se utilizó ácido ascórbico durante el precultivo o la deshidratación osmótica se consiguió disminuir de forma significativa la formación de MDA y la acumulación del radical superóxido (O2•-) en la mayoría los pasos analizados, aunque esta reducción no parece tener un efecto directo en la estabilidad genética del material recuperado. ABSTRACT Cryopreservation has been described as an effective technique for the long term of ex situ conservation that has been successfully applied to numerous species, and is of especial relevance for those with vegetative propagation, infertile or endangered, in which simpler systems of ex situ conservation, such as seed banking, are not feasible. It also has advantages over in vitro conservation, as it reduces or eliminates excessive material handling, avoids periodic subcultures and thus limits the risk of contamination and the appearance of somaclonal variation. However, plant material is subjected to different treatments involved in the cryopreservation procedures, which impose several stresses. Among them, oxidative stress can potentially cause damage to membranes, proteins, carbohydrates and DNA. In this work, two cryopreservation techniques have been evaluated in Mentha × piperita L. shoot tips, sterile hybrid between Mentha aquatica L. and Mentha spicata L. Two genotypes ('MEN 186' and 'MEN 198') were used to compare two techniques: encapsulation-dehydration and droplet-vitrification. The analysis of survival and recovery capacity of the material after the cryopreservation treatments, and the analysis of the genetic stability by molecular markers (RAPD and AFLP) have enabled the comparison between protocols and treatments. The study of the two cryopreservation procedures revealed a higher genetic variability in the encapsulation-dehydration technique, especially in genotype 'MEN 186', as 'MEN 198' was more stable in all analyses. The instability generated in this technique was not exclusive of cryopreserved explants, pretreatments prior to immersion in NL also caused DNA variations. The type of sampled plant material revealed also differences in the stability: callus samples showed greater instability than shoots. Three different culture media were used for the recovery of shoot tips after cryopreservation, using different combinations of growth regulators: "Reed" (0.5 mgL-1 6-benzylaminopurine, BAP), "Senula" (0.5 mgL-1 6-dimetilalilamino-purine, 2-iP + 0.1 mgL-1 α-naphthalene acetic acid, ANA) and "Nodes" (0.5 mgL-1 BAP + 0.1 mgL-1 ANA). "Reed" medium increased survival and recovery of shoot tips in both genotypes and techniques and decreased callus formation. However, it didn`t have a significant effect on genetic stability. "Senula" medium caused a higher genetic stability in the most unstable genotype, 'MEN 186'. To reduce oxidative damage during encapsulation-dehydration, and increase shoot tip recovery and maintain genetic stability, the effect of added antioxidants (ascorbic acid, vitamin E and glutathione) in the shoot tip preculture medium was studied. These treatments had no significant effect on both stability and recovery. To better understand the events during the encapsulation-dehydration process, the effect of each step of the protocol on stability and recovery was evaluated separately. Moreover, the oxidation level was determined by quantifying malondialdehyde (MDA) formation and detecting free radical accumulation (using the thiobarbituric acid assay, and specific fluorescent probes, respectively). The oxidative stress was detected from the first steps and increased throughout the protocol until the immersion in liquid nitrogen. This was also reflected in the gradual decline of recovery and genetic stability. The use of antioxidants (ascorbic acid and vitamin E) in the shoot tip preculture medium had no effect in maintaining genetic stability; only vitamin E increased recovery in one of the steps studied (after desiccation). However, when ascorbic acid was used during the preculture or during the osmotic dehydration, a significantly decrease was observed in MDA formation and superoxide radical accumulation in most of the steps analyzed, although this reduction did not seem to have a direct effect on the genetic stability of recovered material.

Periodic emission of droplets from an oscillating electrified meniscus of a low-viscosity, highly conductive liquid

The generation of identical droplets of controllable size in the micrometer range is a problem of much interest owing to the numerous technological applications of such droplets. This work reports an investigation of the regime of periodic emission of droplets from an electrified oscillating meniscus of a liquid of low viscosity and high electrical conductivity attached to the end of a capillary tube, which may be used to produce droplets more than ten times smaller than the diameter of the tube. To attain this periodic microdripping regime, termed axial spray mode II by Juraschek and Röllgen [R. Juraschek and F. W. Röllgen, Int. J. Mass Spectrom. 177, 1 (1998)], liquid is continuously supplied through the tube at a given constant flow rate, while a dc voltage is applied between the tube and a nearby counter electrode. The resulting electric field induces a stress at the surface of the liquid that stretches the meniscus until, in certain ranges of voltage and flow rate, it develops a ligament that eventually detaches, forming a single droplet, in a process that repeats itself periodically. While it is being stretched, the ligament develops a conical tip that emits ultrafine droplets, but the total mass emitted is practically contained in the main droplet. In the parametrical domain studied, we find that the process depends on two main dimensionless parameters, the flow rate nondimensionalized with the diameter of the tube and the capillary time, q, and the electric Bond number BE, which is a nondimensional measure of the square of the applied voltage. The meniscus oscillation frequency made nondimensional with the capillary time, f, is of order unity for very small flow rates and tends to decrease as the inverse of the square root of q for larger values of this parameter. The product of the meniscus mean volume times the oscillation frequency is nearly constant. The characteristic length and width of the liquid ligament immediately before its detachment approximately scale as powers of the flow rate and depend only weakly on the applied voltage. The diameter of the main droplets nondimensionalized with the diameter of the tube satisfies dd≈(6/π)1/3(q/f)1/3, from mass conservation, while the electric charge of these droplets is about 1/4 of the Rayleigh charge. At the minimum flow rate compatible with the periodic regimen, the dimensionless diameter of the droplets is smaller than one-tenth, which presents a way to use electrohydrodynamic atomization to generate droplets of highly conducting liquids in the micron-size range, in marked contrast with the cone-jet electrospray whose typical droplet size is in the nanometric regime for these liquids. In contrast with other microdripping regimes where the mass is emitted upon the periodic formation of a narrow capillary jet, the present regime gives one single droplet per oscillation, except for the almost massless fine aerosol emitted in the form of an electrospray.

Regimes of spray vaporization and combustion in counterflow configurations

This article addresses the problem of spray vaporization and combustion in axisymmetric opposed-jet configurations involving a stream of hot air counterflowing against a stream of nitrogen carrying a spray of fuel droplets. The Reynolds numbers of the jets are assumed to be large, so that mixing of the two streams is restricted to a thin mixing layer that separates the counterflowing streams. The evolution of the droplets in their feed stream from the injection location is seen to depend fundamentally on the value of the droplet Stokes number, St, defined as the ratio of the droplet acceleration time to the mixing layer strain time close to the stagnation point. Two different regimes of spray vaporization and combustion can be identified depending on the value of St. For values of St below a critical value, equal to 1/4 for dilute sprays with small values of the spray liquid mass loading ratio, the droplets decelerate to approach the gas stagnation plane with a vanishing axial velocity. In this case, the droplets located initially near the axis reach the mixing layer, where they can vaporize due to the heat received from the hot air, producing fuel vapor that can burn with the oxygen in a diffusion flame located on the air side of the mixing layer. The character of the spray combustion is different for values of St of order unity, because the droplets cross the stagnation plane and move into the opposing air stream, reaching distances that are much larger than the mixing layer thickness before they turn around. The vaporization of these crossing droplets, and also the combustion of the fuel vapor generated by them, occur in the hot air stream, without significant effects of molecular diffusion, generating a vaporization-assisted nonpremixed flame that stands on the air side outside the mixing layer. Separate formulations will be given below for these two regimes of combustion, with attention restricted to the near-stagnation-point region, where the solution is self-similar and all variables are only dependent on the distance to the stagnation plane. The resulting formulations display a reduced number of controlling parameters that effectively embody dependences of the structure of the spray flame on spray dilution, droplet inertia, and fuel preferential diffusion. Sample solutions are given for the limiting cases of pure vaporization and of infinitely fast chemistry, with the latter limit formulated in terms of chemistry-free coupling functions that allow for general nonunity Lewis numbers of the fuel vapor.

Stranski-Krastanov InN/InGaN quantum dots grown directly on Si(111)

The authors discuss and demonstrate the growth of InN surface quantum dots on a high-In-content In0.73Ga0.27N layer, directly on a Si(111) substrate by plasma-assisted molecular beam epitaxy. Atomic force microscopy and transmission electron microscopy reveal uniformly distributed quantum dots with diameters of 10–40 nm, heights of 2–4 nm, and a relatively low density of ∼7 × 109 cm−2. A thin InN wetting layer below the quantum dots proves the Stranski-Krastanov growth mode. Near-field scanning optical microscopy shows distinct and spatially well localized near-infrared emission from single surface quantum dots. This holds promise for future telecommunication and sensing devices.

Lattice pulling effect and strain relaxation in axial (In, Ga)N/GaN nanowire heterostructures grown on GaN-buffered Si(111) substrate

Transmission electron microscopy and spatially resolved electron energy-loss spectroscopy have been applied to investigate the indium distribution and the interface morphology in axial (In,Ga)N/GaN nanowire heterostructures. The ordered axial (In,Ga)N/GaN nanowire heterostructures with an indium concentration up to 80% are grown by molecular beam epitaxy on GaN-buffered Si(111) substrates. We observed a pronounced lattice pulling effect in all the nanowire samples given in a broad transition region at the interface. The lattice pulling effect becomes smaller and the (In,Ga)N/GaN interface width is reduced as the indium concentration is increased in the (In,Ga)N section. The result can be interpreted in terms of the increased plastic strain relaxation via the generation of the misfit dislocations at the interface.

Electroreflectance characterization of AlInGaN/GaN high-electron mobility heterostructures

Room temperature electroreflectance (ER) spectroscopy has been used to study the fundamental properties of AlxInyGa${}_{1-x-y}$N/AlN/GaN heterostructures under different applied bias. The (0001)-oriented heterostructures were grown by metal-organic vapor phase epitaxy on sapphire. The band gap energy of the AlxInyGa${}_{1-x-y}{\rm{N}}$ layers has been determined from analysis of the ER spectra using Aspnes' model. The obtained values are in good agreement with a nonlinear band gap interpolation equation proposed earlier. Bias-dependent ER allows one to determine the sheet carrier density of the two-dimensional electron gas and the barrier field strength.

Electron tomography of (In,Ga)N insertions in GaN nanocolumns grown on semi-polar (11(2)over-bar2) GaN templates

We present results of scanning transmission electron tomography on GaN/(In,Ga)N/GaN nanocolumns (NCs) that grew uniformly inclined towards the patterned, semi-polar GaN( 112̄ 2 ) substrate surface by molecular beam epitaxy. For the practical realization of the tomographic experiment, the nanocolumn axis has been aligned parallel to the rotation axis of the electron microscope goniometer. The tomographic reconstruction allows for the determination of the three-dimensional indium distribution inside the nanocolumns. This distribution is strongly interrelated with the nanocolumn morphology and faceting. The (In,Ga)N layer thickness and the indium concentration differ between crystallographically equivalent and non-equivalent facets. The largest thickness and the highest indium concentration are found at the nanocolumn apex parallel to the basal planes.

Electrocatalytic oxidation enhancement at the surface of InGaN films and nanostructures grown directly on Si(111)

Pronounced electrocatalytic oxidation enhancement at the surface of InGaN layers and nanostructures directly grown on Si by plasma-assisted molecular beam epitaxy is demonstrated. The oxidation enhancement, probed with the ferro/ferricyanide redox couple increases with In content and proximity of nanostructure surfaces and sidewalls to the c-plane. This is attributed to the corresponding increase of the density of intrinsic positively charged surface donors promoting electron transfer. Strongest enhancement is for c-plane InGaN layers functionalized with InN quantum dots (QDs). These results explain the excellent performance of our InN/InGaN QD biosensors and water splitting electrodes for further boosting efficiency.